In a known way, a satellite telecommunications system intended to provide a service to fixed and mobile terminals includes, as shown in FIG. 1 for example, a geostationary satellite 10, at least one terrestrial gateway 23 located in a chosen metropolitan coverage area 21, for example a country, illuminated by a beam from a first transmitting and receiving antenna 70 located on board the satellite 10, and a set of user terminals 35 distributed in a service area, for example a side of the Earth visible from the satellite 10. The service area includes a global coverage area 30 and a plurality of theatre of operation coverage areas 31a, 31b, 31c of different shapes and sizes, located in different positions in the global coverage area 30, wherein the theatre coverage areas may be very extensive and may relate to a region, such as the regional area 31c. These different coverage areas, namely the global coverage areas 30 and theatre coverage areas 31a, 31b, 31c, are illuminated by a second antenna system 60 of the satellite 10, which generates a beam covering the global coverage area and an additional beam for each theatre coverage area. Thus, in the example of FIG. 1, the antenna system 60 generates four different beams covering, respectively, the global coverage area 30 and the three theatre coverage areas 31a, 31b, 31c. The set of terminals includes fixed terminals, transportable terminals and mobile terminals, for example land vehicles, ships and aircraft, which are located in the global coverage area and in the theatre coverage areas of the service area, at different positions, on land, at sea and in the air respectively. The gateway 23 communicates with the satellite 10 via a first bidirectional radio link 22, and the user terminals 35 communicate with the satellite 10 via second bidirectional radio links 36. The satellite serves to connect the gateway 23 to the user terminals 35, and to connect the user terminals to each other, by means of radio signals. The traffic between the gateway 23 and the user terminals 23 located in the global and theatre coverage areas is called “star traffic”, while the traffic between two user terminals is called “mesh traffic”. Mesh traffic is inter-spot traffic when it relates to communications between two terminals located in two different coverage areas, whereas mesh traffic is intra-spot (or “loopback”) traffic when it relates to communications between two terminals located in the same coverage area. The theatre coverage areas are geographical areas in which operations take place and where the active terminals are concentrated. These theatre coverage areas can be positioned anywhere within the global coverage area 30 and are of different types, according to their surface area. In the example of FIG. 1, the theatre coverage areas are of three different sizes on the ground, corresponding, respectively, to a narrow spot 31b which may, for example, be 600 km in diameter at the nadir; a theatre spot 31a which may, for example, be 2000 km in diameter at the nadir; and a regional spot 31c which may, for example, be 4000 km in diameter at the nadir. Different frequency channels are allocated to each theatre coverage area 31a, 31b, 31c and to the global coverage area 30. If the links 22 between the gateway 23 and the satellite 10 are in band X, the frequency plan must include another frequency channel allocated to the metropolitan coverage area 21 of the gateway 23.
These known telecommunications systems have a number of drawbacks. A first drawback is the low, or non-existent, rate of frequency re-use. This is because options for re-using the same frequency bands or channels in a number of different coverage areas to increase the total bandwidth capacity of the system are structurally limited or unavailable. On the one hand, the frequency band allocated to the global coverage area 30 cannot be re-used in theatre coverage areas 31a, 31b, 31c or in the metropolitan coverage area 21; on the other hand, the possibility of re-using the same frequency band between two theatre coverage areas depends on the distance between the two coverage areas and the isolation performance between the two corresponding beams generated by the antenna system 60 of the satellite 10. The theatre coverage areas may be located in any positions, are variable in time, and are difficult to predict, because they are dependent on the world geopolitical context, natural disasters, and other factors; consequently there are always cases of operation in which it is impossible to re-use the frequency channels between two different coverage areas. Thus the guaranteed total bandwidth of a telecommunications system for military or governmental use according to the prior art is limited to the total band allocated to the system, for example 500 MHz in the X band between 7 and 8 GHz, and the bandwidth of the frequency channels allocated to the global and theatre coverage areas is also limited, as is the performance of any anti-jamming device based on spread spectrum, direct sequence spread spectrum or frequency hopping spread spectrum techniques in the coverage areas.
A second drawback is the poor intrinsic gain performance of the antenna system of the satellite in the global coverage area and in extensive theatre coverage areas such as regional spots. Thus, regardless of the techniques used to design the satellite antennas, which may be direct radiation active antennas, mechanical reflector antennas, or other types, the gain of the antennas in the global coverage area reaches a ceiling of less than 18 dB at the edge of coverage, while in a theatre coverage area with a diameter of 3000 km the antenna gain reaches a ceiling of less than 30 dB at the edge of coverage. These constraints on the gain performance of the satellite antennas result in a considerable variation of the radio frequency performance of the satellite between one type of coverage and another, and therefore a considerable variation of the quality of the links between the coverage areas for any given type of user terminal. In particular, the very poor performance of the satellite in terms of sensitivity in the global or regional coverage areas limits the available bit rate in the uplink, from the terminal towards the satellite, for small mobile or transportable terminals whose radio frequency performance is limited. Furthermore, the establishment of links from the satellite towards these small fixed or mobile terminals requires a large radio frequency capacity for the payload of the satellite, thus imposing operational limits on the number of links established and on the bit rate of each link.
These drawbacks lead to either a specialization of the terminals according to the coverage area, in which case the performance of the terminal, typically in terms of the antenna diameter, is specified according to the coverage area in which the terminal is to operate, or the overdesign of all the terminals, to enable them to communicate in any type of coverage area. Furthermore, these various drawbacks restrict the possibilities of increasing the total capacity of the telecommunications system in terms of the total bit rate and the number of simultaneously active terminals.